Question

Find the last digit of the number 13 + 23+ 33 + 43… + 993.

Answer 1

here a=13
d=23-13=10
an =993
an =a+(n-1)d
993=13+(n-1)10
(n-1)×10 =993-13
(n-1) = 980/10
n=98+1

n=99